HF linkups offer a non-line-of-sight capability which makes it possible to carry out communications at long or indeed very long distance without requiring recourse to a satellite.
The long-distance communication capability of HF linkups relies on the reflection of HF waves (2, 30 MHz) off the layers of the ionosphere, layers whose qualities are not stable over time and in space, thereby leading to strong variations of the propagation channel. To this instability of the channel are also added the presence of possible jammers, in particular at night where the passing HF spectrum is less significant and therefore more crowded.
Despite its instability, this channel exhibits the benefit of allowing long-distance communications without it being necessary to deploy a complicated or expensive infrastructure beforehand, in contradistinction to satellite communications for example.
Considering also its better stealth with respect to the satellite, this explains why it is currently sought to increase the bitrates offered on HF linkups.
In addition to the most customary uses according to the directives of various standards at frequency bands of width 3 kHz (so-called conventional HF channeling also referred to as SSB, and their standardized extension in the so-called LSB dual mode at 6 kHz known to the person skilled in the art that works on two adjacent channels, a solution has also been proposed in the Applicant's patent application entitled “Method and system for HF band adaptive communications, under the number US 2012/0309330, to consider employing a plurality (n) of such 3 kHz channels (contiguous or otherwise) so as to offer more significant useful bitrates to users of the HF band.
One of the problems to be solved within the framework of the use of a wider frequency band is that of the effective management of the power budget limited by the characteristics of the power amplifier of the transmitter system.
It is clearly apparent that when considering a plurality of channels distributed in a relatively wide band (e.g. 200 kHz), and when carrying out a wideband transmission alone capable of making it possible to attain a high bitrate (bitrates>32 kb/s), the various pathways distributed over this wide band will not see the same imperfections of the propagation channel: typically the fading will be different, and a fortiori the jamming, intentional or otherwise, will differ.
It is known from the prior art to use one and the same modulation scheme, for example, M-PSK, M-QAM known to the person skilled in the art, for two pathways, where the same code and interleaver is shared and finally where the power budget is equidistributed. One of the drawbacks of the prior art is of working on a typical value of two pathways, and of not having any means for optimizing the number of carriers or any indication of power distribution.
Patent application WO 2007/015962 discloses a method for performing multi-band transmissions.
Patent application US 2006/291582 describes a solution of adaptation to selective channels for transmission in a multiple input, multiple output system, better known by the abbreviation “MIMO”. The idea is to use power adaptation to allow all the channels to see the same equivalent signal-to-noise ratio and therefore to use the same modulation for all the channels.
By taking account of a system comprising a maximum of n carriers corresponding to n pathways distributed in a given frequency band, the method according to the invention will seek to obtain the best distribution of the power budget available on these n carriers, so as to optimize the useful bitrate transmitted.
The technical problem posed is therefore, for a given power budget, to determine the corresponding optimal number of pathways and/or the corresponding choice of modulation and of coding, as well as the power distribution on each of the pathways so as to allow optimization of the highest possible useful bitrate, corresponding to the given operating point of the system.
The expression “given operating point” of a system is known to the person skilled in the art and corresponds to an operating point fixed by the user. This operating point is defined as the value of the signal-to-noise ratio at which provision is made to operate the system.
The useful bitrate is defined as the ratio of the number of user bits provided as input to the corrector code to the time required for their transmission. Similarly, the total bitrate is defined as the ratio of the total number of bits transmitted (coded bits and signaling bits) relative to the time required for their transmission.